1. Field of the Invention
This invention relates to a measuring instrument for measuring the thickness or the height of an upper face of a wafer such as a semiconductor wafer held on a chuck table equipped on a processing machine for a wafer such as a laser beam machine or laser processing machine.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of regions are partitioned by scheduled division lines called streets disposed in grating-like arrangement on the surface of a semiconductor wafer having a substantially disk shape, and such a device as an IC or an LSI is formed in each of the partitioned regions. Then, the semiconductor wafer is cut along the streets to cut the regions in which the devices are formed to divide individual semiconductor chips.
In order to achieve down-scaling and enhancement in function of an apparatus, a module structure wherein a plurality of semiconductor chips are laminated and electrodes of the laminated semiconductor chips are connected has been put into practical use. The module structure is configured such that, at a position on the surface of a semiconductor wafer at which an electrode called bonding pad is formed, a hole (via hole) is formed from the rear face such it extends to the bonding pad, and a conductive material such as aluminum is filled in the via hole such that it connects to the bonding bad (refer, for example, to Japanese Patent Laid-Open No. 2003-163323). The via hole provided in the semiconductor wafer described above is formed using a drill. However, the via hole formed in the semiconductor wafer is as small as 100 to 300 μm in diameter, and perforation by drilling has a problem that the productivity is low.
In order to eliminate the problem described above, a laser beam machine has been proposed which includes machining feed amount detection means for detecting a relative machining feed amount between a chuck table for holding a workpiece and laser beam irradiation means, storage means for storing X and Y coordinate values of fine holes to be formed in the workpiece, and control means for controlling the laser beam irradiation means based on the X and Y coordinate values of the fine holes stored in the storage means and a detection signal from the machining feed amount detection means, a laser beam being irradiated when the X and Y coordinate values of a fine hole to be formed in the workpiece reach a position just below a condenser of the laser beam irradiation means (refer, for example, to Japanese Patent Laid-Open No. 2006-247674).
Meanwhile, as a method of dividing a semiconductor wafer or the like described above along a street, also a laser machining method has been attempted wherein a pulse laser beam of a wavelength having permeability in a wafer is used and irradiated with the convergence light point thereof adjusted to the inside of a region to be divided. A dividing method which uses the laser machining method includes the steps of irradiating a pulse laser beam of an infrared region having permeability in a workpiece with the convergence light point thereof adjusted to the inside from the rear face side of a wafer, continuously forming a degenerated layer along a street in the inside of the wafer, and applying external force along the street at which the strength is deteriorated by the formation of the degenerated layer to divide the wafer along the street (refer, for example, to Japanese Patent No. 3,408,805).
In the above-described formation method of irradiating a pulse laser beam from the rear face of a semiconductor wafer to form a via hole, the irradiation of the pulse laser beam must be stopped appropriately so that a bonding pad formed on the surface of the semiconductor wafer may not be perforated. To this end, it is necessary to irradiate a predetermined number of pulses of the pulse laser beam corresponding to the thickness of the semiconductor wafer. However, the thickness of the semiconductor wafer has a dispersion, and therefore, it is significant to confirm the thickness of the semiconductor wafer at which each bonding pad is positioned.
Further, in the laser machining method wherein a pulse laser beam of a wavelength having permeability in a wafer is used and irradiated with the convergence light point thereof adjusted to the inside of a region to be divided thereby to form a degenerated layer along a street in the inside of the semiconductor wafer as described above, it is desirable to form the degenerated layer uniformly at a predetermined depth in the inside of the semiconductor or the like. However, if the semiconductor wafer has undulation and has a dispersion in thickness, then when a laser beam is irradiated, a degenerated layer cannot be formed uniformly at a predetermined depth from a relationship of the refractive index. Accordingly, in order to form a degenerated layer uniformly at a predetermined depth in the inside of a semiconductor layer, it is necessary to detect unevenness of a region in which a laser beam is to be irradiated in advance and cause laser beam irradiation means to follow up the unevenness to machine the semiconductor wafer.